1. Field of the Invention
The present invention relates to a nitrogen dioxide (NO.sub.2) absorbent or adsorbent for automotive exhaust gas purifying facilities to remove NO.sub.2 by absorption or adsorption from ventilation gas discharged from road tunnels which contains nitrogen oxides (NO.sub.x) in low concentrations.
2. Description of the Related Art
A conceivable common way of removing by absorption NO.sub.2 (which is an acidic gas) from NO.sub.x -containing gas is by reaction with alkali for fixation into nitrate or nitrite. In actual practice, however, very little NO.sub.2 is absorbed when air containing several ppm of NO.sub.2 is bubbled in an aqueous solution of KOH.
In contrast, NO.sub.2 in very low concentrations as above can be absorbed and removed effectively by the use of a solid absorbent consisting of a porous carrier (such as titania or alumina which has solid acid properties) and KOH impregnated into and supported on the carrier, which had previously been proposed by the present inventors. (See Japanese Patent Laid-open No. 211427/1998.)
The present inventors studied in detail the nitrogen dioxide absorbent composed of a carrier of solid acid or activated carbon and a hydroxide of strong alkali (such as K and Na) supported thereon. As the result, they found the following problems involved in it.
In the ase of a carrier of solid acid:
(1) A nitrogen dioxide absorbent composed of a carrier of solid acid and a hydroxide of strong alkali (such as K and Na) supported thereon is low in absorption rate unless NO.sub.x contain s nitrogen monoxide (NO) in an amount equal to or more than NO.sub.2. See FIG. 1. PA0 (2) If NO.sub.x contains NO in an amount equal to or more than NO.sub.2, the absorbing rate of NO.sub.2 is approximately linearly proportional to its concentration at 10 ppm or above. (Incidentally, the absorbing rate of NO is constant regardless of its concentration under the same absorbing condition.) However, in concentrations at 5 ppm or below, the absorbing rate of NO.sub.2 begins to decrease with decreasing concentration; it is very low at 1 ppm or below. This tendency becomes remarkable according as more NO.sub.2 is accumulated in the absorbent. See FIG. 2. PA0 (3) If NO.sub.x contains NO in an amount equal to or more than NO.sub.2 and if NO.sub.x is not accumulated initially in large amounts on the surface of the absorbent, NO and NO.sub.2 are absorbed almost equally and hence the absorbent is rapidly consumed. PA0 (4) Activated carbon (well known as an adsorbent of NO.sub.2) adsorbs NO.sub.2 alone and adsorbs and absorbs NO.sub.2 efficiently even in the absence of NO. However, its absorbing rate decreases at 1 ppm or below according as the accumulated amount of NO.sub.2 increases. PA0 (5) Activated carbon releases as much NO as one-half to one-quarter the amount of NO.sub.2 adsorbed. PA0 (6) An absorbent composed of activated carbon and a strong alkali hydroxide supported thereon absorbs NO and NO.sub.2 almost equally in the initial stage, like an absorbent employing an acid solid as the carrier; however, its absorbing rate of NO decreases according as the accumulated amount of NO.sub.x increases on the surface of the absorbent. After a large amount of NO.sub.x has accumulated, it reversibly releases as much NO as one-half to one-quarter the amount of NO.sub.2 absorbed. This results in an increase in NO concentration in the gas phase. See FIG. 3. PA0 (1) NO.sub.2 in low concentrations does not react directly with alkali. PA0 (2) At first, NO.sub.2 is adsorbed to the carrier. The adsorbed NO.sub.2 then changes into a compound which readily reacts with alkali. Finally, this compound reacts with alkali for its fixation. PA0 (3) Presumably, the compound in (2) readily reactive with alkali is N.sub.2 O.sub.3 in the case of solid acid carrier supporting strong alkali, and it is N.sub.2 O.sub.4 in the case of activated carbon carrier supporting strong alkali. EQU NO+NO.sub.2.fwdarw.N.sub.2 O.sub.3 EQU 2NO.sub.2.fwdarw.N.sub.2 O.sub.4 PA0 (4) In either case, those compounds in (3) decompose into nitric acid or nitrate (which is stable) and nitrous acid or nitrite (which is unstable). EQU N.sub.2 O+2MOH.fwdarw.2MNO.sub.2 +H.sub.2 O EQU N.sub.2 O.sub.4 +2MOH.fwdarw.MNO.sub.3 +MNO.sub.2 +H.sub.2 O PA0 (5) The nitrous acid or nitrite is oxidized into nitric acid or nitrate (which is stable) or decomposed into NO, which is released. EQU 2MNO.sub.2 +O.sub.2.fwdarw.2MNO.sub.3 EQU 2MNO.sub.2 +H.sub.2 O.fwdarw.NO.sub.2 +2MOH+NO (released) PA0 (6) Usually, strong alkali nitrite is not readily decomposed but is oxidized into nitrate, and weak alkali nitrite is readily decomposed into NO. PA0 (a) The absorbent should have a catalytic action to denature NO.sub.2 into a compound readily reactive with alkali. PA0 (b) The absorbent should have alkali densely arranged around the active site of the catalyst in (a) so that the denatured product of NO.sub.2 reacts with alkali. PA0 (c) The resulting alkali nitrate and nitrite in (b) should have moderate stability so that it fixes the adsorbed NO.sub.2 in a stable manner and permits it to diffuse rapidly outward from the vicinity of the active site for NO.sub.2 denaturation in (a). PA0 (1) the solid acid carrier or activated carbon carrier provided with the denaturing catalytic action on NO.sub.x, and PA0 (2) the basic amino acid and organic amine compound or alkali hydroxide supported on the carrier.
In the case of a carrier of activated carbon:
The present inventors have interpreted these phenomena as follows.
(where, M: alkali metal)
It is concluded from the foregoing discussion that a d esirable nitrogen dioxide absorbent should meet the following requirements.
The concept mentioned above is depicted in FIG. 4.
NO.sub.2 in the gas phase is adsorbed at the active sites of the catalyst and is denatured there into a form readily reactive with alkali. The denatured product rapidly reacts with alkali hydroxide near the active site, and the resulting nitrate and nitrite are retained stably. The active site of the catalyst becomes vacant, and the cycle of adsorption-denaturation can be repeated.
The thus formed nitrate or nitrite has its anions (NO.sub.3.sup.- or NO.sub.2.sup.-) dispersed into the vicinity from near the active site. Thus free alkali is regenerated near the active site, and it repeats its reaction with the denatured product forming at the active site.
At an early time when the absorption of NO.sub.2 has just begun, alkali is present in large amounts near the active site and hence the rate of absorption is limited by the rate of absorption of NO.sub.2 to the active site. Accordingly, as the amount of NO.sub.2 absorbed increases, the rate of absorption is limited by the rate of diffusion into the vicinity of the active site.
The present inventors had previously proposed a carrier of titania (TiO.sub.2) impregnated with a manganese (Mn) salt, followed by drying and firing. This carrier meets the requirement of (a), or it has a catalytic action for denaturation of NO.sub.2. See Japanese Patent Laid-open No. 192049/1996.
The present inventors had also proposed a nitrogen dioxide absorbent employing a carrier impregnated with a hydroxide of alkali metal (such as K and Na). See Japanese Patent Laid-open No. 211427/1998.
The above-mentioned absorbent was capable of efficient absorption of NO.sub.2 in low concentrations (1 ppm or below). This was a remarkable improvement as expected. However, it was found that the rate of absorption of NO.sub.2 in low concentrations rapidly decreases according as NO.sub.2 accumulates on the surface of the absorbent. See FIG. 5. A probable reason for this is that strong alkali nitrate or nitrite is so stable that it does not permit its anions (NO.sub.2.sup.- or NO.sub.3.sup.-) to readily diffuse from near the active site, with the result that free alkali rapidly decreases near the catalytic active site.